Electronic strobe

ABSTRACT

An electronic strobe circuit for flash cameras or flash attachments in which a capacitive energy storage circuit is discharged into a flash tube to provide illumination in synchronism with the opening of the camera shutter. The strobe includes a charging circuit for gradually charging the storage circuit toward a predetermined charge level sufficient for illuminating the flash tube to a desired intensity upon opening of the shutter and a charge limiting circuit including a balanceable bridge coupled to the storage circuit and a detecting device for inhibiting operation of the charging circuit whenever the voltage across the flash tube reaches a desired level. The charge limiting circuit causes the storage circuit to be operated in an intermittent or pulsating manner when it reaches the desired flash voltage, the pulsating nature of the limiting circuit being additionally utilized to provide an audio-visual indication to the operator when the storage circuit is fully charged.

United States Patent [191 arpol [451 Jul 2,1974

[ ELECTRONIC STROBE I Zvi Y. Karpol, Bronx, NY.

[73] Assignee: Berkey Photo, Inc., Paramus, NJ.

[22] Filed: Oct. 11, 1972 [2]] Appl. No.: 296,628

[75] Inventor:

Primary Examiner-Herman Karl Saalbach Assistant ExaminerRichard A. Rosenberger Attorney, Agent, or FirmNeuman, Williams, Anderson & Olson [5 7] STRA CT An electronic strobe circuit for flash cameras or flash attachments in which a capacitive energy storage circuit is discharged into a flash tube to provide illumination in synchronism with the opening of the camera shutter. The strobe includes a charging circuit for gradually charging the storage circuit toward a predetermined charge level sufficient for illuminating the flash tube to a desired intensity upon opening of the shutter and a charge limiting circuit including a balanceable bridge coupled to the storage circuit and at detecting device for inhibiting operation of the charging circuit whenever the voltage across the flash tube reaches a desired level. The charge limiting circuit causes the storage circuit to be operated in an intermittent or pulsating manner when it reaches the desired flash voltage, the pulsating nature of the limiting circuit being additionally utilized to provide an audiovisual indication to the operator when the storage circuit is fully charged.

7 Claims, 3 Drawing Figures 4 is 9 a 32 1 ELECTRONIC STROBE This invention relates generally to camera flash units, and more particularly concerns electronic flash units providing repetitive flash operations from a single battery-powered light source.

Electronic strobes or flash units have become popular as an alternative to flash bulbs or cubes of the disposable variety because of the economy of operation and compactness inherent in the strobe unit. Light sources have improved to the point where high intensity flash of a reliable and constant nature can be obtained repetitively from a single gaseous discharge lampfor the life of the camera.

Typically, the known strobe units employ a flash tube filled with a rare gas as a light source. While such tubes provide an instantaneous light intensity comparable to that of self-destructive flash bulbs, they require an instantaneous voltage supply on the order of 300-2,000 volts D.C., far in excess of the voltage available from conventional replaceable batteries.

Prior strobe units have employed conversion circuits for slowly building up the voltage acros a storage device, such as a large capacitor, from a low voltage source, such as batteries providing approximately 312 volts D.C. Voltage step-up is most efficiently provided by a transformer. Therefore, the low voltage D.C. source is used to power an oscillator which provides the A.C. signal necessary for voltage step-up in a transformer. The AC. output of such a conversion circuit is then rectified and utilized to charge the storage capacitor.

The approach described has proven feasible in certain cameras; however, it has been difficult to obtain a constant intensity flash in these cameras due to the inherently poor regulation of the flash voltage. Specifically, prior strobe units have either failed to provide the operator with an indication of the state of the flash volt age or, where such an indication has been provided, the regulation of the flash voltage has been so poor as to make it impossible to accurately predict the brightness of the flash in any particular instance. Since the exposure or lens opening of the camera is ideally set in accordance with the flash intensity, it has been difficult in the past to accurately coordinate flash intensity and lens opening when a strobe is used.

It is an object of the present invention to provide a strobe unit which is inherently more reliable and which provides a more constant flash than the strobe units heretofore known in the art. It is a related object of the present invention to provide an'electronic flash unit utilizing a conventional flash tube and energy storage device, but with a highly improved flash voltage regulating circuit which insures that the voltage across the storage device, and hence the intensity of the flash, are constant through many operations.

A more specific object of the present invention is the provision of a flash unit utilizing a highly sensitive bridge detector circuit for regulating the high voltage across a flash tube.

It is a further object of the invention to provide with such a bridge-detector circuit a pulsating charge disabling device which allows the voltage across the flash tube to vary in an intermittent fashion within a very limited range around the desired constant flash voltage.

More specifically, it is an object of the invention to provide means for allowing intermittent charging and discharging of the storage element in a regular, repetitive fashion about its desired operating point.

It is still another object of the present invention to utilize the above-described repetitive nature of the capacitor voltage regulator for providing a visual and/or audible signal to the operator indicating that the flash unit is ready.

Still another object of the present invention is the provision of an electronic flash unit in which the storage capacitor is utilized in a voltage multiplying circuit operating in conjunction with a DC. to A.C. converter.

Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which:

FIG. I is a perspective view of a camera incorporating an electronic strobe unit constructed in accordance with the present invention;

FIG. 2 is a schematic representation of the electronic flash control circuit utilized in the strobe unit of FIG. l; and

FIG. 3 is a diagram of the strobe synchronization circuit.

While the invention will be described in connection with the preferred embodiment, it will be understood that I do not intend to limit the invention to that embodiment. On the contrary, I intend to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention.

Turning first to FIG. 1, there is shown a camera of the instant-picture type. The camera has a housing It) generally encompassing a film carrying compartment i2 and lens-supporting portion 14. An adjustable focusing lens 16 protrudes from the front of the housing 10, while an auxiliary opening 18 lies adjacent the lens 16 to provide an input port for the ambient light to a sh utter control device described generally in the copending application of Israel Nesson, Ser. No. 296,678, now US. Pat. No. 3,762,289 issued Oct. 2, 1973, filed concurrently herewith and assigned to the asignee of the present application.

For loading the spring mechanism (not shown) of the camera shutter, there is provided a cocking lever 20 protruding from the side of the lens portion 141 of the housing. The cocking lever, when actuated, places the shutter mechanism in readiness for opening during the exposure period in a manner well known to those skilled in the art. A picture is taken by releasing the shutter mechanism so as to cause an instantaneous opening thereof. To this end, trigger button 22 is provided, which may be mechanically coupled to the shutter mechanism by any of numerous techniques currently known to the art.

For normal daylight photography the sun or artificial room lighting may prove sufficient for proper exposure. However, for those instances in which the ambient light is insufficient there is provided an electronic flash or strobe unit which forms a part of a strobe light and viewfinder assembly indicated generally at 24. The left part of the assembly 24 comprises the viewfinder and consists of various lenses for properly framing the scene to be photographed. The remaining volume of the assembly 24 is the electronic strobe, which comprises a lens 26, behind which there is a reflector, a

high intensity flash tube (not visible) and the associated strobe circuitry shown in detail in FIG. 2. At the rear of the assembly 24 and not visible in the view of FIG. 1 is an indicator light for signalling the readiness of the flash circuit in a manner to be hereinafter described.

Turning now to FIG. 2, there is shown on the far right side thereof a flash tube 30 having positive and negative supply terminals 32 and 34, respectively, and a trigger terminal 36. The flash tube 30 is a gas-discharge tube for producing high-intensity, short-duration flashes of light. Typically, the tube 30 is filled with a rare gas, such as Xenon, and the tube supply terminals 32, 34 are respectively termed the anode and cathode terminals. The tube is flashed by applying a high voltage A.C. trigger signal to the trigger electrode or tennina] 36 simultaneously with the application of a highvoltage DC. signal across the supply terminals 32 and 34. Preferably, the supply voltage is as high as possible, typically in the area of 3002,000 volts"D.C.

Such a high DC. voltage is not available from conventional replaceable batteries. Therefore, the supply voltage is generally derived from an oscillatorconverter circuit operating in conjunction with a voltage step-up transformer. The oscillator-converter circuit provides a trickle charge at a high voltage to an energy storage circuit connected across the supply terminals of the tube 30. In the present instance, the energy storage circuit is in the form of a large-valued capacitor 40 connected across the supply terminals 32, 34 of the tube 30. It has been found that a 600 microfarad capacitor rated at 360 volts and 40 watt-seconds is well suited for this purpose.

For charging the capacitor 40 to the desired high voltage level, there is provided a charging circuit indicated generally at 42. The charging circuit 42 includes an oscillator 44, a step-up transformer 46 and a voltage multiplying circuit encompassing the storage capacitor 40 and further including a charge transfer capacitor 48 and directional diodes 50 and 52 connected effectively in series with the storage capacitor 40.

The oscillator 44 is powered by a low-voltage D.C. source 56, typically consisting of a plurality of zinc carbon batteries connected in series. The active element of the oscillator is an PNP transistor 58, having its emitter terminal connected directly to the positive supply voltage and its collector terminal connected to the negative supply voltage through a primary winding 60 on the transformer 46. The base voltage of the transistor 58 is controlled by the bias normally provided by the series circuit consisting of a pair of resistors 62 and 64 in the collector circuit of a transistor 66, the emitter of the transistor 66 being returned to the negative supply through a bias resistor 68. As will be later described, the transistor 66 is normally in the on or conducting mode so as to provide a forward bias for the baseemitter junction of the transistor 58. Oscillation of the device is produced as a result of feedback to the base circuit of the transistor 58 via the inductive coupling of flux from the primary winding 60 to a feedback secondary winding 70 connected in series with the base circuit of the transistor 58. Connected between the positive source terminal and the junction of resistors 62 and 64 is a capacitor 72 which maintains conduction through the transistor 66 at a substantially constant level during voltage variations across the feedback winding 70. Constructed as shown, the circuit 44 oscillates as a result of the 180 phase relation between the output voltage developed across the winding 60 and the feedback voltage developed across the winding 70.

For the purpose of providing a high voltage A.C. signal for charging the storage capacitor 40, the transformer 46 is provided with a secondary winding 74 having turns numbering far in excess of the turns of the primary winding 60 so as to effect a step-up voltage trans formation. At this point it is noted that the operating frequency of the oscillator 44 and the magnitude of the voltage developed across the secondary winding 74 increase as the charge on the capacitor 40 accumulates.

In accordance with one aspect of the present invention, the storage capacitor 40 is an integral part of a voltage multiplying circuit which includes the transfer capacitor 48 and the directional diodes 50 and 52. In operation, positive half cycles of the AC. signal across the second winding 74 result in a direct charge of the capacitor 40 as a result of the conduction through a series circuit completed through the directional diode 50 and the transfer capacitor 48. During negative half cycles of the voltage across the winding 74 the directional diode 50 is reverse biased and no charge is transferred to the capacitor 40. However, the diode 52 is forward biased during the negative half cycle, causing conduction through and charging of the capacitor 48 at the indicated polarity. During the next positive half cycle, the directional diode 52 becomes reverse biased, and charging of the transfer capacitor 48 stops. As the voltage across the secondary winding 74 goes positive, it adds to the voltage accumulated across the transfer capacitor 48 during the previous negative half cycle. This results in a voltage doubling effect, and the resulting voltage presented across the capacitor 40 through the diode 50 has a magnitude which is approximately twice that of the transformer secondary voltage alone.

Viewed in another manner, energy is transferred from the secondary winding 74 of the transformer 46 to the storage capacitor 40 during positive half cycles. During negative half cycles energy is transferred from the secondary winding 74 to the transfer capacitor 48. The energy accumulated in the transfer capacitor 48 is additionally transferred to the storage capacitor 40 during the positive half cycle of the transformer secondary voltage.

In accordance with another aspect of the present invention, means are provided for limiting the charge delivered to, and the voltage developed across, the storage capacitor 40 by inhibiting operation of the converter oscillator 44 whenever the voltage across the storage capacitor 40 reaches a predetermined level sufficient for illuminating the flash tube to a desired intensity upon opening the shutter. To this end, there is provided a balanceable bridge circuit coupled to the storage capacitor 40 and adapted to be unbalanced to produce an output of a first polarity whenever the voltage across the storage capacitor 40 reaches a predetermined level. Additionally, there is provided threshold detecting means for inhibiting operation of the oscillator 44 whenever the output of the bridge circuit is of the first polarity. Bridge circuits have been found to be particularly well suited for the present application because of their inherent sensitivity to small changes in their parameters. In the present instance, the left side of the bridge consists of two legs, an upper leg consisting of a resistor and a lower leg consisting of a fixed resistor 82 and a variable resistor 84. Similarly, the

right side of the bridge has an upper leg which includes a resistor 86 and a lower leg which includes a zener diode 88 in parallel with a smoothing capacitor 90. The two sides of the bridge are effectively connected in parallel across the storage capacitor 41) and flash tube 30. The output signal of the bridge is measured between the junction points of the upper and lower legs of the respective sides of the bridge. In other words, a first terminal 92 of the bridge outputis defined by the junction between resistors 80 and 82 in the left side of the bridge, while a second terminal 94 of the bridge output is defined by the junction of the resistor 86 and zener diode 88. The zener diode 88 acts as a voltage clamping device in the bridge. When its reverse breakdown voltage is reached, no further build-up of voltage across the diode will occur. Similarly, resistors 80, 82 and are chosen so as to provide a voltage-dividing effect across the flash tube 30 which causes the voltage at the output terminal 92 of the bridge to begin to exceed the clamped voltage at the terminal 94 shortly before the voltage across the flash tube 30 reaches the predetermined level. Further increase in the voltage across the flash tube 30 results in a change of polarity for the bridge output terminal 92 with respect to the bridge output terminal 94. In other words, at the predetermined charge level of the storage capacitor 40, the voltage at the terminal 92 begins to go positive with respect to the voltage at terminal 94.

For responding to the change in polarity of the bridge circuit output there is provided a threshold detecting circuit consisting of first and second transistors 96 and 66 biased and so interconnected so as to form a Schmitt trigger circuit across the bridge output terminals 92, 94. The collector of the transistor 96 drives the base of the transistor 66, the emitters of both transistors being connected to the bridge terminal 94 through a threshold-establishing resistor 68. The transistor 66 is normally biased into conduction by current flowing from the positive side of the source 56 through a bias resistor 98. Current flowing through the collector-emitter circuit of the transistor 66 establishes a voltage drop across the resistor 68 which determines the threshold of the circuit. The base terminal of the transistor 96 is coupled to the output terminal 92 of the bridge circuit. As such, the base-emitter junction of the transistor 96 remains reverse biased so long as the voltage at the terminal 92 does not exceed the voltage at the terminal 94 by more than the threshold voltage across the resistor 68. In actual practice, the voltage across the resistor 68 is on the order of 0.5-1.0 volts. Therefore, it is seen that the detecting circuit is effectively activated upon a change of polarity at the output terminals of the bridge circuit.

Such a change of polarity effectively forward biases the base-emitter junction of the transistor 96 so as to begin conduction through the collector-emitter circuit thereof. As the transistor 96 goes into saturation, the transistor 66 stops conducting. As a result, forward bias is removed from the base-emitter junction of the oscillator transistor 58 and further operation of the charging circuit 42 is inhibited for repetitive periods of time, the duration of which is dependent on the value of capacitor 100 and the threshold of the detector circuit.

A further element effecting the operation of the threshold detecting circuit is the presence of a capacitor 100 connected across the output terminals 92, 94 of the bridge circuit. The threshold detecting circuit is activated only after the capacitor 180 is charged to the threshold voltage existing across the resistor 68. Upon activation of the threshold circuit, the capacitor 108 is discharged through the base-emitter junction of the transistor 96 and the resistor 68. As such, the repetition rateof the charge-leakage cycle is additionally determined by the capacitor lltlll. lt has been found that optimum performance of the flash unit is achieved when the components are chosen so as to provide, as full storage voltage, intermittent charging at a repetition rate of approximately Kr-5 Hz.

In accordance with another aspect of the present invention, there is provided means for monitoring the charge level of the capacitor 40, and hence, the voltage across the flash tube 30, so as to provide a pulsating indication to the operator whenever the voltage from the storage capacitor reaches the desired maximum operating level. More specifically, a visual indication of the full-charge condition is effected through the provision of an auxiliary light source 1114 connected in series with a current limiting resistor 196 across the secondary winding 74 of the charging circuit transformer 46. As indicated previously, the amplitude of the AC. signal at the secondary winding 74 increases with increasing voltage across the storage capacitor 48. The light source 104 is chosen to provide illumination when the voltage supplied to it is slightly less than percent the storage voltage for the flash tube 30. A neon-filled gaseous discharge tube is ideally suited for this purpose in view of its firing voltage of approximately 150-l 80 volts. As the charging circuit 42 brings the voltage level across the storage capacitor 40 toward its maximum level, the light source 104 is illuminated to an essentially constant intensity. As the storage voltage reaches its maximum level, the charge limiting circuit described above becomes operative to cause intermittent operation of the charging circuit 42. As a result, the light source 104 flashes on and off in synchronism with the charge-discharge cycle of the capacitor 40, indicating the achievement of a fully charged condition by the capacitor.

ln addition to the visual indication of full-charge there is provided means for generating an audible indication of the fully charged condition of the capacitor 40. Specifically, the transformer 46 of the charging circuit 42 is of the ferrite core type and operates as a magnetostrictive transducer for providing an audible tone or hum in response to excitation in the audible frequency range. As a result, it has been found that a suitable audible indication of full-charge is provided by the intermittent operation of the oscillator 44 during the repetitive charge-discharge sequence attendant to the achievement of the predetermined maximum voltage across the storage capacitor 411. The intermittent illumination of the auxiliary light source 104 occurs simultaneously with the audible tone from the transformer 46.

To this point the description has been directed to the provision of a supply voltage for the anode and cathode terminals 32, 34 of the flash tube 30 and to the circuitry for developing and regulating that voltage. In using the strobe unit with the camera, it is necessary to provide means for generating a trigger signal for the flash tube trigger terminal 36 so as to cause the storage capacitor 40 to discharge through the flash tube 30. To this end, there is provided in the circuit of FIG. 2 a trigger circuit 108 which includes a trigger capacitor 110 connected in a series circuit with a resistor 112 between the positively charged terminal of the storage capacitor 40 and the trigger transformer 114. As the voltage across the storage capacitor 40 increases, so also does the energy stored in the capacitor 110 increase. For converting the energy stored in the capacitor 110 into a high-voltage A.C. signal to trigger the flash tube 30, there is provided a transformer 114 having a primary winding 116 connected in series with the capacitor 110 during normal charging thereof. A secondary winding 118 of the transformer 114 provides an output trigger signal to the trigger terminal 36 of the tube 30. Since the trigger signal is to be generated at the beginning of the camera exposure, there is provided a switching device 122 which is closed in response to depression of the trigger button 22 (FIG. 1) as a result of a mechanical interaction represented by the dotted line 124. Closure of the switching device 122 connects the charged trigger capacitor 110 directly across the primary winding 116 of the transformer 114. The resulting L-C circuit rings at its resonant frequency, causing inductive coupling of an A.C. signal to the transformer secondary winding 118 which is connected to the trigger terminal 36 of the flash tube 30. The turns-ratio of the transformer 114 is such that a high-voltage signal is applied to the tube trigger terminal 36 to cause discharge thereof.

In brief review of the strobe operation, power is initially applied to the strobe circuit by the operator as switch 28 is closed. Assuming that the storage capacitor 40 is completely uncharged, the oscillator 44 of the charging circuit 42 begins to oscillate and the amplitude of the A.C. signal across the secondary winding 74 of the transformer 46 gradually increases. This signal drives a voltage doubler circuit consisting of the transfer capacitor 48 and diodes'50 and 52 so as to increase the voltage across the storage capacitor 40. As the voltage across the capacitor 40 reaches its desired maximum level, the output signal across the terminals 92, 94 of the bridge changes polarity and charges the capacitor 100. As the charge on the capacitor 100 builds up, the voltage across it exceeds the threshold voltage of the threshold detecting circuit, causing the transistors 96 and 66 to change their states of conduction. The transistor 66 stops conduction and inhibits further operation of the oscillator 44. The charging circuit is disabled, causing a slow discharge of the capacitor 40 to begin. As the voltage across the capacitor 40 falls to a predetermined differential below its desired maximum voltage, the energy stored in the capacitor 100 is discharged through the now-conducting transistor 96 and the next charging cycle begins. The charging and discharging of the capacitor 40 results in intermittent operation of the charging circuit 42, resulting in a pulsating audible tone being produced by the magnetostrictive effect of the transformer 46. Simultaneously, intermittent flashing of the auxiliary light source 104 occurs to signal the operator that the flash unit is ready. The camera user generates a flash by closing the switch 122 via the trigger button 22. Energy previously stored on the capacitor 110 is transferred to the primary winding 116 of the transformer 114 to cause ringing therewith, resulting in a burst of A.C. energy at the trigger input 36 to the flash tube 30.

When used in conjunction with an integrating type shutter, switch 122 will be closed when the shutter fully opens, discharging the energy previously stored in the trigger capacitor into the trigger transformer 114, and the integrating capacitor C in FIG. 3.

The part of the energy that is transferred from capacitor 110 into C will make C, appear as if the integration has been completed and correspondingly cause the shutter to close.

In most shutters, the mechanical delay from the time the solenoid is released to the time the shutter starts to cover the lens, is long enough for the strobe light to complete its function, if not, an additional R.C. type delay may be added.

A more detailed description of FIG. 3 is found in US. Pat. No. 3,762,289, issued Oct. 2, 1973, and entitled CAMERA SHUTTER ASSEMBLY, of Israel Nesson, filed concurrently herewith, which I incorporate by reference herein.

I claim as my invention:

1. In an electronic camera flash unit having a capacitive energy storage circuit which is discharged into a flash tube in synchronism with the opening of the camera shutter, the combination comprising:

a direct current source;

a charging circuit coupled to said source for gradually charging said storage circuit toward a predetermined charge level sufficient for illuminating said flash tube to a desired intensity upon opening of said shutter;

charging limiting means including: a) a balanceable bridge circuit having opposite corners thereof coupled across said storage circuit and adapted to be unbalanced to produce an output signal whenever the charge in said storage circuit rises past said predetermined level and b) means operatively associated with said bridge circuit for alternately deactivating and reactivating said charging circuit in response to the presence or absence, respectively, of said output signal; and

charge monitoring means operatively associated with said charging circuit for providing an audible tone whenever said charging circuit is active.

2. In an electronic camera flash unit the combination according to claim 1 further including means for leaking charge from said storage circuit during deactivation of said charging means so as to effect alternating deactivation and reactivation of said charging circuit after the charge level of said storage circuit reaches said predetermined charge level, whereby said audible tone pulsates in synchronism with the charge-discharge cycle of said storage circuit.

3. In an electronic camera flash unit, the combination according to claim 2 wherein said charge monitoring means includes an auxiliary light means coupled to said charging circuit and operative to provide a repetitive visual signal in synchronism with said audible tone.

4. In an electronic camera flash unit, the combination according to claim 3 wherein the repetition rate of pulsating tone and light is approximately /2 5 Hz after said storage circuit achieves said predetermined charge level.

5. In an electronic camera flash unit, the combination according to claim 1 wherein said charging includes a DC. to A.C. converter consisting of a) an oscillator selectively coupled to said direct current source for providing an A.C. signal, b) step-up transformer means associated with said oscillator for providing a high voltage A.C. signal of a peak amplitude at least an order of magnitude greater than the voltage of said direct current source, and c) means for coupling said high voltage A.C. signal to said storage circuit so as to increase the charge level thereof.

6. In an electronic camera flash unit, the combination according to claim 21 wherein said step-up transformer means is a magnetostrictive transducer providing an audible tone during operation of said charging circuit.

7. In an electronic camera flash unit having a capacitive energy storage circuit which is discharged into a flash tube in synchronism with the opening of the camera shutter, the combination comprising:

a direct current source;

a charging circuit coupled to said source for gradually charging said storage circuit toward a predetermined charge level sufiicient for illuminating said falls below said predetermined level.

l 1: l l

I UNITED STATES PA'IEIEI'I oFFICE @ERTIFICATE OF CORRECTION mmm mm 3,322,393 Dated July 2, 1974 limvemtofls) ivi Y. Karool v 5 I it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 18, "second" should be secondary Column. 6, line 9, "as" (second occurrance) should be at Claim 1 (folumn 8, line 28, "charging" should be charge Claim 6 @olumm 9, line 5, "21" should be 5 sighed and sealed this 5th day of November 1974.

1 (SEAL) Attest:

iMcCQY Mo GIBSUN JR C. MARSHALL DANN Attegzstimg @fificem I Commissioner of Patents roan poaoso (no-s9) USCOMWDC 603w, i UJ. GOVIINIIIIT PIIII'IING OFFICE: "ll 0-llil Patent No $822,393 Dated July 2 1974 g "j UNITED STATES PATEPEIJT OFFICE it is certified that error appears in the above-identified patent and ithat said Letters Patent are hereby corrected as shown below:

Column 4, line 18, "second" should be secondary Column, line 9, "as" (second occurrance) should be at @laim l (3011mm line 28, "charging" should be charge Claim 6 7 law 9, line 5, "21" should be 5 sighed and sealed this 5th day of November 1974.

(SEAL) Arrest:

wmcoy Me mason JPN c. MARSHALL DANN Arresting @fficer Commissioner of Patents roan Po-wso (IO-69) 1 USCOMWDC 6037M, 9 UJ. OOVIIHIINT PRINTING OIFICI: I!" 0-in-1 

1. In an electronic camera flash unit having a capacitive energy storage circuit which is discharged into a flash tube in synchronism with the opening of the camera shutter, the combination comprising: a direct current source; a charging circuit coupled to said source for gradually charging said storage circuit toward a predetermined charge level sufficient for illuminating said flash tube to a desired intensity upon opening of said shutter; charging limiting means including: a) a balanceable bridge circuit having opposite corners thereof coupled across said storage circuit and adapted to be unbalanced to produce an output signal whenever the charge in said storage circuit rises past said predetermined level and b) means operatively associated with said bridge circuit for alternately deactivating and reactivating said charging circuit in response to the presence or absence, reSpectively, of said output signal; and charge monitoring means operatively associated with said charging circuit for providing an audible tone whenever said charging circuit is active.
 2. In an electronic camera flash unit the combination according to claim 1 further including means for leaking charge from said storage circuit during deactivation of said charging means so as to effect alternating deactivation and reactivation of said charging circuit after the charge level of said storage circuit reaches said predetermined charge level, whereby said audible tone pulsates in synchronism with the charge-discharge cycle of said storage circuit.
 3. In an electronic camera flash unit, the combination according to claim 2 wherein said charge monitoring means includes an auxiliary light means coupled to said charging circuit and operative to provide a repetitive visual signal in synchronism with said audible tone.
 4. In an electronic camera flash unit, the combination according to claim 3 wherein the repetition rate of pulsating tone and light is approximately 1/2 - 5 Hz after said storage circuit achieves said predetermined charge level.
 5. In an electronic camera flash unit, the combination according to claim 1 wherein said charging includes a D.C. to A.C. converter consisting of a) an oscillator selectively coupled to said direct current source for providing an A.C. signal, b) step-up transformer means associated with said oscillator for providing a high voltage A.C. signal of a peak amplitude at least an order of magnitude greater than the voltage of said direct current source, and c) means for coupling said high voltage A.C. signal to said storage circuit so as to increase the charge level thereof.
 6. In an electronic camera flash unit, the combination according to claim 21 wherein said step-up transformer means is a magnetostrictive transducer providing an audible tone during operation of said charging circuit.
 7. In an electronic camera flash unit having a capacitive energy storage circuit which is discharged into a flash tube in synchronism with the opening of the camera shutter, the combination comprising: a direct current source; a charging circuit coupled to said source for gradually charging said storage circuit toward a predetermined charge level sufficient for illuminating said flash tube to a desired intensity upon opening of said shutter; and charge limiting means including: a) a zener regulated balanceable bridge circuit having opposite corners thereof coupled across said storage circuit and adapted to be unbalanced to produce an output which goes to a first polarity from a second polarity as the charge in said storage circuit rises past said predetermined level, b) means responsive to said unbalance of a first polarity for deactivating said charging circuit, and c) means associated with said deactivating means for reactivating said charging circuit when the charge level of said storage circuit falls below said predetermined level. 